I was trying to use System.out.println to help with debugging and I found that it wasn't printing to the console. On inspection I found that my program had created 4 output consoles ( one for Java DB processes, one for the DB server, one for debugging the program, and one for program output ). I found my expected println in an unexpected console - the DB server output.
I would like to get a handle on these outputs. I expected the System class to have a list field of active output consoles ( printstreams ), something like :
ArrayList<PrintStream> getActivePrintOutputs()
But I don't see one. How do I get them?
System has no concept of multiple output streams beyond those specified by out and err, and you can access those by just referencing System.out and System.err respectively.
If there are other consoles or output streams being used, they must have been created by other points in your code (or other points in a library's code that you're using.)
Normally you have only one active System.out output stream, so there is no reason for the system to maintain a list.
If you want to trace all the PrintStreams created you can use instrumentation to track their creation, or put a break point in the constructor for the class and debug your program.
NOTE: Is is normal for a program to create multiple logs files for different purposes and these might be the PrintStreams you are thinking of.
Related
I have made a nice UI with three different logs (a general log and two class specific ones).
Every log can print different lines with different colors.
I was thinking of doing this so I can show info/errors/warnings.
Now, the thing is, that I'd like to have detailed debug only when I set a variable (something like detailedDebug = true).
I'd like something like this:
Simple | Detailed
Error thrown in ... | Error thrown.. + dump of all variables related to the error
Now, with if statements I can achieve that easily, but, that seems overly complicated (complicating the code for debugging reasons too).
How could I implement this (while making it easy to use and most importantly clean)?
Should I make a method in every class that uses the logging features that automatically checks for a variable then does what asked?
You should use the the log level as the variable to control the detail. When you want more detail, turn the level down to FINEST.
Hoewever, some operations that you wish to log might require considerable resources to calculate the detail (example, you may retrieve info from the DB, etc). In this case you should use if statements because the resources will be consumed even if the log level is at ERROR level.
Example :
The following code will always execute :
logger.log(Level.FINEST, "Some detailed log info which sows the results from DB {0}",
new Object[]{ getResults() });
If you only want to execute this code when you are showing FINEST, you need to wrap the statement in an if statement :
if (logger.isLoggable(Level.FINEST)) {
// Some intensive logging
}
I'm using the commoncrawl example code from their "Mapreduce for the Masses" tutorial. I'm trying to make modifications to the mapper and I'd like to be able to log strings to some output. I'm considering setting up some noSQL db and just pushing my output to it, but it doesn't feel like a good solution. What's the standard way to do this kind of logging from java?
While there is no special solution for the logs aside of usual logger (at least one I am aware about) I can see about some solutions.
a) if logs are of debug purpose - indeed write usual debug logs. In case of the failed tasks you can find them via UI and analyze.
b) if this logs are some kind of output you want to get alongside some other output from you job - assign them some specail key and write to the context. Then in the reducer you will need some special logic to put them to the output.
c) You can create directory on HDFS and make mapper to write to there. It is not classic way for MR because it is side effect - in some cases it can be fine. Especially taking to account that after each mapper will create its own file - you can use command hadoop fs -getmerge ... to get all logs as one file.
c) If you want to be able to monitor the progress of your job, number of error etc - you can use counters.
Hi I need to show the result in a memo instead using System.out.println, but isn't possible to put the stdout in a list for example and after display the contents of this list in swing memo because I need to display every line of the result in real time or when it is showed.
I'm think in something that works like an observer of System.out.println and when some data or information have been printed in the console I want to be able to capture it and display in a memo.
For a better comprehension, I execute some commands remotely in an unix server and retrieve the results of these commands in the stdout and compute some time and metrics with them, and definitely I need to do in this way to simulate the behavior of an remote application.
The solution could be a way to show every line or every item in the list in the memo in the exactly time that it is produced.
And the swing memo can deal with big strings, more than 500kb or more than 1MB?
Because the entire result printed in the sdtout in my ID is really huge.
Thx
See Message Console, for which the description states:
There may be times when you want to capture output from your program and display it for the user. This is generally done by creating a console. Using Swing it is not too difficult to create a simple console using a JTextArea or JTextPane. Our message console will be able to display output written to System.out and System.err. ..
Message Console Screenshot
The JTextPane form of Message Console in append mode.
(Screenshot obtained from the linked article at Rob Camick's 'Java Tips Weblog'.)
You can find what you are looking for here :
The webpage is in French, but still code, is quite clear and in java.
Basically, they use to threads to read from 2 PipedOutputStream. One of them is plugged on System.out, the second one is plugged on System.err. When something is available in one of the pipedOutputStream, they write it in the widget.
On standard console all things are printed in white whether we have written it in System.out or System.err. In IDE(for me Eclipse) we can see different color output on console for both. i.e. black for System.out and red for System.err.
Is System.err is only provided for use in IDEs? Cause on cmd we can not distinguish System.out and System.err. Both are printed in same color.
These are two different output streams that are available in most of OS's. You don't have them color coded due to settings of your terminal/command line environment. On the other hand your IDE provides different visualization for different streams.
If you wanted to color them, consider using ANSI escape sequences.
This is a relict from the unix world, where most functionality is available as unix commands which were intended to be chained. The output of one command is used to feed another like here:
grep -i 'token' file | mail peter#address.de
The pipe symbol only redirects the stdout (System.out), but not the stderr (System.err). So error messages would be seen on the console, and the regular output would go to the mail command.
If there were just one stream, one could not distinguish between them.
Windows, not relying on the command line (This changed in Windows Server 2008!) didn't invent again but just took the unix concepts and made them available in their dos commands, too. It is just that nearly no Windows only users usually know what they are good for.
From system-in-out-error:
System.err is a PrintStream.
System.err works like System.out
except it is normally only used to
output error texts. Some programs
(like Eclipse) will show the output to
System.err in red text, to make it
more obvious that it is error text.
From JLS:
20.18.3 public static PrintStream err;
The initial value of this variable is
a "standard" error output stream,
already open and ready to accept
output data. Typically, this
corresponds to display output or
another output destination specified
by the host environment or user. By
convention, this output stream is used
to display error messages or other
information that should come to the
immediate attention of a user even if
the principal output stream, the value
of the variable out, has been
redirected to a file or other
destination that is typically not
continuously monitored. Note that this
field is not final, so its value may
be updated if necessary.
From Java World 02-qa-1220-console.html
Other post in Stackoverflow coloring-text-with-java-in-windows
System.out goes to the standard output stream (stdout) and System.err goes to the standard error stream (stderr). See standard streams for details and how you can control where they go. Eclipse just conveniently colour codes them for you so you can distinguish them in one view.
Both System.out and System.err always exist in Java.
Depending on your console it might be possible to get it to display the two streams in a different colour.
Example use:
try {
Class.doSomething(myFile);
} catch (Exception e){
System.err.println("Fatal error performing doSomething: " + e);
System.exit(-1);
}
I have a legacy PSVM application which I'd like to redirect its logging output to unique files per execution. So, if I invoke it at 10:00, then have it redirect it's output to {thread-id}-10:00.log; and another thread of execution may begin an execution at 10:01, and its output would go to {thread-id}-10:01.log. I understand that this is not elegant.
My questions are:
is this possible?
does someone have an idea of how to approach?
is it possible to release/destroy an appender when it's no longer needed?
Thanks!
I would start with FileAppender and derive from that to create your own. Simply modify your version to get the current thread id and append a suitable thread-id/timestamp to the file prior to creation. You would maintain (say) a map of (buffered) FileWriters keyed on thread id.
Writing appenders is quite trivial - here's a Javaworld guide on how to do it.
In the above, is it at all likely that your program will start up twice in one minute ? Would you want to append a process id or similar to maintain uniqueness ?
It is not possible, at least not easy to do in log4j. However, if you look at SiftingAppender shipping with logback (log4j's successor), it is designed to handle the creation of appenders on runtime criteria as well as their removal when no longer needed.
If you application needs to create just one log file per application launch, you could simply name your log file based on a timestamp. Shout on the logback-user mailing list if you need further assistance.